Developer interviews

At my current company, we've been trying to hire a new back-end PHP developer for some time. As the senior engineer on staff, and the only back-end person, it's my job to handle the technical portion of the screening process, including resume screening and technical interview. Unfortunately, I have lots of other work to do, so I never had time to come up with a really good procedure to test candidates. In addition, most of the interviews are over the phone with people in other cities, which makes it difficult to have them write code without some advanced preparation - which, again, I haven't had time to do.

So, as a compromise, I came up with a standard list of questions that I ask candidates. I originally wrote this list with the intention of using it for junior developers (1 to 3 years of experience), thinking that anyone with 5+ years of experience would breeze through it. However, I've found that a disturbingly large percentage of more experienced developers fumble more than one of the questions. Unfortunately, I'm not yet sure if that's just because we're getting poor quality applicants, or if it's because my standards are too high.

Therefore, in the interests of helping my fellow developers - both those looking for new opportunities and those who are interviewing the first group - as well as in the hopes of getting some good feedback, I present my list of interview questions. Note that I do not include answers. There are two reasons for this:
1) There is not always a single "right" answer - it is perfectly valid for the question to turn into a conversation.
2) If you're an interviewer and you don't already know the answer, you shouldn't be asking the question; if you're an interviewee and you don't know the answer, you should look it up and learn something new.
With each question I've included some notes on the question's intent. Some of these are conceptual, some are simply tests of knowledge, and some are spring-boards to further discussion.

  1. Solve the FizzBuzz problem. How would you solve it in an object-oriented fashion? Using recursion?
  2. OK, I hardly ever ask this question; you really need to be in the same room to make it work. However, it's still useful to see if the candidate can program and to see how they think. For the object-oriented and recursive versions, I'm not so much looking for the right answer as I am for the approach - and I tell them as much. In particular, I'm looking to see where the candidate goes with the open-ended requirement for object-orientation. Do they go for procedural code that just happens to use some objects? Or do they go straight to polymorphic classes, maybe with a factory pattern thrown in?
  3. Define "inheritance" and "polymorphism". Give an example of when you might use each.
  4. This is your "OOP 101" question. Any half-competent developer should be able to describe inheritance and give the stock "shape" or "animal" example. Polymorphism is a bit more slippery - I often don't get a good answer to that part of the question. In fact, a lot of people seem to think that polymorphism is basically the same thing as method overloading and don't really make the connection to inheritance.
  5. What is the difference between an "abstract class" and an "interface"?
  6. I actually ask this because one of our products is built primarily in FLEX, and interfaces abound in our ActionScript code. With this, I'm really interested in seeing if the candidates understands the idea of explicitly specifying a contract in the code, as opposed to relying on, e.g., PHP's loose typing to make things "just work".
  7. What is "SQL injection" and how might you protect against it? What about XSS/CSRF?
  8. This is, of course, the basic security question. On the SQL injection side, I'm specifically looking for awareness of prepared statements, e.g. in MySQLi or PDO. More generallly, I'm looking for an awareness of filtering an validation. On the script injection side, I like to see what candidates say they use to filter input. An awareness of standard escaping and sanitizing function (strip_tags(), htmlentities(), etc.) is always good - knowledge of the filter extension gets bonus points.
  9. In JavaScript, what does it mean to say that functions are "first class" or "first order" objects?
  10. This is a lead-in to determine how much, if anything, the candidate knows about the more functional aspects of JavaScript - anonymous functions, closures, etc. Surprisingly (at least to me), hardly anyone seems to be familiar with the terminology of "first class" entities. So far I've only had two people know what I was talking about right off the bat.
  11. What is the difference between NULL and the empty string in SQL?
  12. This one is more of a trivia question to test the depth of knowledge of SQL. Bonus points if the candidate mentions SQL's three-valued logic in the follow-up question: "What is the result of the comparison NULL = ''"?
  13. What is an "index" in a database and how would you decide what columns to index?
  14. This one is just to check that the candidate has some basic knowledge of database performance considerations.
  15. Given a sample HR database that contains the following tables:
  16. Employees (employee_id INTEGER PRIMARY KEY, employee_name TEXT, ...)
    Departments (department_id INTEGER PRIMARY KEY, department_name TEXT, ...)

    What would you have to add to the schema to associate employees with departments?

    This is the basic SQL competency question. The typical answer is to add a foreign key from Employees to Departments, at which point I spring the "what if an employee needs to be in two departments" scenario and see if they come up with a join table. Also, it gives a good opportunity to see if they understand when to use inner versus outer joins. It's surprising how many people struggle with this question.
  17. Describe a project you worked on that was particularly enjoyable or interesting. What did you like about it?
  18. I stole this from an interview I did once where I was the applicant. It can be enlightening to see what catches a person's interest. If nothing else, it can help you get an idea of where their head is; are they a hard-core CS person, a touchy-feely UI person, or something else entirely?

Personally, I consider this list to be inadequate at best. But as I said, I haven't had time to develop an adequate test and this is at least better than asking "So how well do you know X?", if for no other reason than it's harder to BS an acceptable answer. Any thoughts or opinions are welcome.

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Comments #

    No Subject

    Great post Peter! Was helpful for some recent hiring we did!

    • Comment posted on Friday 8 Apr 2011 at 3:29pm
    • By Jason Martorana
    • (http://bright.com)

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